As typically seen in a liquid crystal display device, a thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon. Thin film transistors manufactured using amorphous silicon has low field effect mobility, but can be formed over a larger glass substrate. In contrast, thin film transistors manufactured using crystalline silicon has high field effect mobility, but due to a crystallization step such as laser annealing, the transistors are not always suitable for being formed over a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique for by which a thin film transistor is manufactured using an oxide semiconductor and such a transistor is applied to an electronic appliance or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor for an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.